The mainstream consensus is that the New Testament was written in a form of Koine Greek, which was the common language of the Eastern Mediterranean from the Conquests of Alexander the Great (335 -- 323 BC) until the evolution of Byzantine Greek (c. 600).

Based on this passage, what was the original language of the new testament? Let's give stream of consciousness first:
To answer this question, we should know that: The mainstream consensus is that the New Testament was written in a form of Koine Greek, which was the common language of the Eastern Mediterranean from the Conquests of Alexander the Great (335 -- 323 BC) until the evolution of Byzantine Greek (c. 600).
So, the final answer is a form of Koine Greek.